Lightweight asymmetric magnet arrays with mixed-phase magnet rings

ABSTRACT

A magnet array includes multiple magnet rings and a frame. The multiple magnet rings are positioned along a longitudinal axis and coaxially with the longitudinal axis, wherein at least two of the magnet rings include mixed-phase magnet rings that are phase-dissimilar. The multiple magnet rings are configured to jointly generate a magnetic field along a direction parallel to the longitudinal axis of at least a given level of uniformity inside a predefined inner volume. The frame is configured to fixedly hold the multiple magnet rings in place.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/772,638, filed Nov. 29, 2018, and U.S. Provisional PatentApplication 62/780,272, filed Dec. 16, 2018, whose disclosures areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to magnet assemblies, andparticularly to lightweight magnet assemblies comprising permanentmagnets and design methods thereof.

BACKGROUND OF THE INVENTION

Designs of permanent magnet arrays aiming at achieving a strong anduniform magnetic field have been previously reported in the patentliterature. For example, U.S. Pat. No. 7,423,431 describes a permanentmagnet assembly for an imaging apparatus having a permanent magnet bodyhaving a first surface and a stepped second surface which is adapted toface an imaging volume of the imaging apparatus, wherein the steppedsecond surface contains at least four steps.

As another example, U.S. Pat. No. 6,411,187 describes adjustable hybridmagnetic apparatus for use in medical and other applications includes anelectromagnet flux generator for generating a first magnetic field in animaging volume, and permanent magnet assemblies for generating a secondmagnetic field superimposed on the first magnetic field for providing asubstantially homogenous magnetic field having improved magnitude withinthe imaging volume. The permanent magnet assemblies may include aplurality of annular or disc like concentric magnets spaced-apart alongtheir axis of symmetry. The hybrid magnetic apparatus may include a highmagnetic permeability yoke for increasing the intensity of the magneticfield in the imaging volume of the hybrid magnetic apparatus.

U.S. Pat. No. 10,018,694 describes a magnet assembly for a magneticresonance imaging (MRI) instrument, the magnet assembly comprising aplurality of magnet segments that are arranged in two or more rings suchthat the magnet segments are evenly spaced apart from adjacent magnetsegments in the same ring, and spaced apart from magnet segments inadjacent rings. According to an embodiment, a plurality of magnetsegments is arranged in two or more rings with the magnetizationdirections of at least some of the magnet segments being unaligned witha plane defined by their respective ring, to provide greater controlover the resulting magnetic field profile.

U.S. Pat. No. 5,900,793 describes assemblies consisting of a pluralityof annular concentric magnets spaced-apart along their axis of symmetry,and a method for constructing such assemblies using equiangular segmentsthat are permanently magnetized.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a magnet array includingmultiple magnet rings and a frame. The multiple magnet rings arepositioned along a longitudinal axis and coaxially with the longitudinalaxis, wherein at least two of the magnet rings are mixed-phase magnetrings that are phase-dissimilar. The multiple magnet rings areconfigured to jointly generate a magnetic field along a directionparallel to the longitudinal axis of at least a given level ofuniformity inside a predefined inner volume. The frame is configured tofixedly hold the multiple magnet rings in place.

In some embodiments, two of the at least two mixed-phase magnet ringscontain only a single permanent magnetic phase with a magnetizationvector in a direction different by more than 45 degrees from oneanother.

In some embodiments, each magnet ring has a rotational symmetry withrespect to an in-plane rotation of the magnet ring around thelongitudinal axis.

In an embodiment, at least one of the magnet rings encircles thepredefined inner volume, wherein a minimal inner radius of the magnetrings positioned on one side of a center of the inner volume along thelongitudinal axis is different from the minimal radius of the magnetrings positioned on the other side of the center of inner volume. Inanother embodiment, the inner volume is an ellipsoid of revolutionaround the longitudinal axis.

In some embodiments, the magnet rings are arranged with reflectionalasymmetry with respect to the longitudinal axis.

In some embodiments, a given magnet ring is made of one of a singlesolid element and an assembly of discrete magnet segments.

In some embodiments, the magnet ring is pre-magnetized with a respectivemagnetization direction that maximizes uniformity of the magnetic fieldinside the inner volume. In other embodiments, the magnet ring ispre-magnetized with a respective magnetization direction that minimizesa fringe field outside the magnet array.

In an embodiment, the discrete magnet segments are electricallyinsulated from each other. In another embodiment, each of the discretemagnet segments has a shape that is one of a shape of sphere, acylinder, an ellipsoid and a polygonal prism. In yet another embodiment,the discrete magnet segments are separated of each other by at least onenon-magnetic element including a solid, gas or liquid.

In some embodiments, the magnet rings have a shape including one of anellipse, a circle and a polygon.

In some embodiments, each of the mixed phase rings has a discreterotational symmetry of at least an order eight.

In an embodiment, the magnet array further includes one or moreadditional arrays of magnet rings, wherein the magnet rings in theadditional arrays are coaxial with respective longitudinal axes that areset at respective angles from the longitudinal axis.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for producing a magnet array, the methodincluding positioning multiple magnet rings along a longitudinal axisand coaxially with the longitudinal axis, wherein at least two of themagnet rings are mixed-phase magnet rings that are phase-dissimilar,with the multiple magnet rings configured to jointly generate a magneticfield along a direction parallel to the longitudinal axis of at least agiven level of uniformity inside a predefined inner volume. The multiplemagnet rings are fixedly held in place using a frame.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an asymmetric magnet array comprising afirst magnet assembly and a second magnet assembly, according to anembodiment of the present invention;

FIGS. 2A and 2B-2D are a perspective view of an asymmetric magnet array,and plots of magnetic field lines generated separately and jointly bythe assemblies, respectively, according to another embodiment of thepresent invention;

FIG. 3 is a perspective view of a segmented magnet ring, which may beany one of the rings in the magnet arrays of FIGS. 1 and 2, according toan embodiment of the present invention;

FIG. 4 is a perspective drawing of a magnet array of phase-dissimilarmixed-phase magnet rings (MPMRs), according to an embodiment of thepresent invention;

FIG. 5 is a plot of magnetic field lines generated by the magnet arrayof FIG. 4, according to an embodiment of the present invention; and

FIG. 6 is a perspective view of a mixed-phase magnet ring (MPMR), whichmay be any one of the rings in the magnet array of FIG. 4, according toan embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Magnetic fields that are strong and uniform are needed in a wide varietyof disciplines, spanning medicine, aerospace, electronics, andautomotive industries. As an example, magnets used in Magnetic ResonanceImaging (MRI) of the human brain typically provide a magnetic field witha strength of 0.1 to 3 Tesla, which is uniform to several parts permillion (ppm) inside an imaging volume of approximately 3000 cubiccentimeters, e.g. the interior of a sphere of radius 9 cm. However, suchmagnets have limited applications due to their considerable size andweight. Moreover, in general with magnet designs, there is a severelylimiting trade-off between weight, magnetic field uniformity, and a sizeof a volume inside which a given uniformity can be achieved.

Embodiments of the present invention that are described hereinafterprovide lightweight permanent magnet arrays that generate strong anduniform magnetic fields (e.g., in the range of 0.1 to 1 Tesla). Some ofthe disclosed magnet arrays are configured for emergency-care brainmobile MRI systems, such as a head MRI system inside an ambulance.Generally, however, the disclosed techniques can be applied in any othersuitable system.

In the description herein, using a cylindrical reference frameconsisting of longitudinal (Z), radial (r), and azimuthal (θ)coordinates, an inner volume is defined as a volume of an ellipsoid ofrevolution around the longitudinal axis. Examples of an inner volume area prolate having its long axis along the longitudinal axis, and anoblate having its short axis along the longitudinal axis. A lateralplane is further defined as any r-θ plane (i.e., a plane orthogonal tothe longitudinal z-axis). A particular definition of an inner volume isan imaging volume of an MRI system inside which the magnetic field hasat least a given level of uniformity.

In some embodiments of the present invention, a magnet array is providedthat comprises a frame, which is configured to hold, fixed in place,multiple magnet rings coaxial with a central longitudinal axis atdifferent positions along the axis, wherein the magnet rings lie inlateral planes with at least one ring encircling an area contained in aninner volume through which the longitudinal axis passes (i.e., the ringintersects the inner volume). In the present description a frame isdefined by its mechanical capability to hold the rings in place, andwhich can be made in various ways, for example, using a yoke or byembedding the rings in a surrounding material (e.g., in epoxy).

The multiple magnet rings are arranged with reflectional asymmetry withrespect to the longitudinal axis. In the context of the presentdisclosure and in the claims, the term “reflectional asymmetry withrespect to the longitudinal axis” means that no plane perpendicular tothe longitudinal axis is a plane of symmetry for the magnet array. Inother words, the magnet array is not symmetric under flipping withrespect to the longitudinal axis at any point along the axis.Reflectional asymmetry is also referred to as point asymmetry ormirror-image asymmetry. For brevity, any reference to “asymmetry” of themagnet array in the description below means the reflectional asymmetrydefined above.

The multiple magnet rings are configured to jointly generate a magneticfield along a direction parallel to the longitudinal axis of at least agiven level of uniformity inside the inner volume. The magnet array haseach magnet ring generate a magnetic field having a rotational symmetry(continuous or discrete) with respect to an in-plane rotation of thering around the longitudinal axis.

In some embodiments, each of the magnet rings of any of the disclosedmagnet arrays has a shape comprising one of an ellipse, most commonly acircle, or of a polygon. The magnet rings are each made of either asingle solid element or an assembly of discrete magnet segments. Themagnet rings are pre-magnetized with a magnetization direction which isdesigned to maximize the uniformity of the magnetic field inside theinner volume and optionally minimize the safety zone defined by the areaaround the magnet for which the magnetic field exceeds 5 gauss.

In some embodiments, which are typically configured for head MRIapplications, a disclosed asymmetric permanent magnet array can bedescribed as comprising a first magnet assembly, comprising two or moremagnet rings having a first inner diameter, and a second magnetassembly, comprising two or more magnet rings having a second innerdiameter. The first inner diameter is larger than the maximal lateraldiameter of the imaging volume and the second inner diameter is smallerthan or equal to the maximal lateral diameter of the imaging volume.

Typically, the magnet rings lie in different longitudinal axispositions. The second magnet assembly is asymmetrically placed relativeto the imaging volume. The asymmetric structure of the disclosed magnetarray is thus optimized to fit a human head, in which physical access toan inner volume (which is the same as the imaging volume) containing thebrain is through the first assembly but not the second. The first andsecond magnet assemblies are configured to jointly generate a magneticfield parallel to the longitudinal axis of at least a given level ofuniformity inside the inner volume.

In some embodiments, the asymmetric magnet array is provided with atleast two mixed-phase permanent magnet rings that are phase-dissimilar.In the context of the present invention, a mixed-phase magnet ring(MPMR) is defined as a magnet ring comprising multiple, repeatingsegments, each of which consisting of two or more phases, at least oneof which is comprised of a permanent magnetic material.

A phase is defined as an element characterized by a particularcombination of (i) material composition, (ii) geometric shape andrelative position within the segment, and (iii) magnetization state. Themagnetization state is represented by three components of magneticmoment, M=(M_(r),M_(θ),M_(z)), which are shared by corresponding phasesin different segments, in the aforementioned cylindrical reference frameof coordinates. The materials of the various phases may be (but notlimited to) permanent magnets, ferromagnetic, ferrimagnetic,paramagnetic, diamagnetic, antiferromagnetic or non-magnetic. The totalmagnetic field of an MPMR at any point is calculated by superposing thecontributions of all phases in the ring which have nonzero values of M.

The phases fill the entire MPMR effective volume, which is defined asthe volume of a polygonal annular ring of a minimum cross-sectionalarea, which just encloses all magnetic phases in the ring. Thevolumetric ratio of a phase is defined as the ratio of the phase volumeto the effective volume of the MPMR.

Two MPMRs are said to be phase-similar if there is a one-to-onecorrespondence between the phases of the two rings for which (a) thevolumetric ratios of corresponding phases are the same, (b) the magneticpermeabilities of corresponding non-permanent magnet phases are thesame, and (c) the magnetization vectors of corresponding phases differat most by a rotation through a constant angle in the r-Z plane commonfor all phases, and by a constant scaling factor in the magnetizationmagnitudes common for all phases.

Thus, when two MPMRs are phase-dissimilar, the relative contribution ofeach individual phase in a given ring to the total magnetic field ofthat ring is different for the two rings. For example, with the aid ofcomputerized magnetic field simulation tools, the phases of at least twoMPMRs which are phase-dissimilar, and the magnetic moment directions oftheir permanent magnet phases, can be adjusted, or “tuned,” so as tooptimize the uniformity of the total magnetic field inside an innervolume. These extra degrees of freedom are most advantageous when thearray is subject to various geometric constraints (such as position ofthe rings, radial/axial thickness), which commonly arouse frommechanical r or manufactural limitations.

It will be appreciated that a solid magnet ring piece can be magnetizedin an azimuthal repetitive manner so as to create repeating segments,with each segment magnetized with a different magnetization directionand/or strength. In the present context, such a magnet piece will beconsidered an MPMR where the phases share common material compositionbut differ in their magnetization states, even though mechanically thereis no actual segmentation of the magnet ring. The same holds, forinstance, for a solid magnet ring created with different materialcompositions where the composition changes in an azimuthal repetitiveway. In such a case, different magnetic compositions areas will beconsidered as different phases. The same holds for a solid magnet piecewhich has its axial thickness and/or radial thickness and/or crosssection geometry vary azimuthally in a repetitive way. In this case thering will be considered an MPMR with phases which differ by theirgeometry but share a common composition and magnetic state, even thoughthere is no segmentation mechanically.

For a given weight of an asymmetric magnet ring array, using two or morephase-dissimilar MPMRs will result in a level of field uniformity insidethe inner volume that is substantially higher than that achieved by theasymmetric array incorporating only one MPMR or several phase-similarMPMRs.

The various types of magnet rings disclosed above are typically made ofa strongly ferromagnetic material, such as an alloy of neodymium, iron,and boron (NdFeB), whose Curie temperature is well above the maximumambient operating temperature. Other material options include ferrites,samarium-cobalt (SmCo) magnets, or any other permanent magnet material.Depending on the design and type of ring, ring segments may have theshape of a sphere, a cylinder, an ellipsoid, or a polygonal prism withshapes such as a cuboid, a wedge, or an angular segment.

The two disclosed techniques to realize magnet arrays (e.g., using anasymmetric geometry, using two or more MPMR rings), separately orcombined, enable the use of strong and uniform magnet arrays inapplications that specifically require lightweight magnet solutions.

FIG. 1 is a perspective view of an exemplary asymmetric magnet array 100comprising a first magnet assembly 110 and a second magnet assembly 120,according to an embodiment of the present invention. As seen, first andsecond magnet assemblies 110 and 120 each comprise at least two magnetrings which are coaxial with a central longitudinal axis, denoted“Z-axis,” which passes through an inner volume 130. The multiplicity ofmagnet rings has variable transverse dimensions and variabledisplacements along the Z-axis. In FIG. 1, by way of example, firstassembly 110 is shown as consisting of four magnet rings, 111-114, andsecond assembly 120 is shown as consisting of four magnet rings,121-124. Each of the rings in assemblies 110 and 120 is either a solidring or a segmented ring, i.e., a ring comprising discrete segments. Thesegments may have the shape of a sphere, a cylinder, an ellipsoid, or apolygonal prism, preferably cuboids. It will be appreciated that therings may have any cross section including non-regular shape crosssection. All segments belonging to a single ring share a common shapeand material composition, as well as the same magnetic moment componentsin the longitudinal (Z), radial, and azimuthal directions. However, oneor more of these characteristics may differ from one ring to another.

In case of a segmented ring, referring to the magnetic moment of asegment means that the segment is uniformly magnetized to a specificdirection in space, its radial, longitudinal and azimuthal directionsare calculated in the segment center of mass.

In case of a solid ring, M varies continuously in space havingazimuthal, radial, and longitudinal components independent of theazimuth coordinate. It will be appreciated that a solid magnet piecewith a complex shape may be magnetized in a fashion that M_(r), M_(θ),or M_(z) changes as a function of Z, or R, in a gradual or stepped way,creating effectively several rings from a magnetization perspective,although mechanically composed of one continuous piece. In the presentcontext, this sort of implementation is regarded as having multiplerings where their borders are determined by the magnetizationperspective, rather than by mechanical segmentation.

The peripheral shape of the rings may be any closed curve, such as acircle, ellipse, or polygon. In some cases, the choice of peripheralshape depends upon the cross-sectional shape of inner volume 130. Itwill be appreciated that a rotational symmetry of a ring, implies amongothers, that its peripheral shape is also rotationally symmetric (Forexample a shape of a circle, or an equiangular-equilateral polygon). Inthe special case where all rings are circular, the minimal inner radiusof rings 111-114 of first assembly 110 is denoted by R1, and the minimalinner radius of rings 121-124 of second assembly 120 is denoted by R2.For a given target radius Ri, which, by way of example, has the lateralradius 140 of inner volume 130 that defines a maximal radius of aspheroid volume inside that is used for imaging and which the magneticfield has at least a given level of uniformity, the values of R1 and R2satisfy the relationship Ri<R1, and 0≤R2≤Ri. In the case of R2=0, atleast one of the rings of second assembly 120 is a solid disc. It isappreciated that assembly 120 may contain rings with inner radius largerthan R2 and even larger than R1. The assemblies are separated in the Zdirection with a gap which is typically (but not limited to) 0-10 cm.For the present purpose, if a ring extends in Z direction to bothassemblies, one part of the ring will be considered as included in thefirst assembly while the other part in the second assembly. In this casethe gap between arrays will be 0.

In an embodiment, in the asymmetric array, the minimal radius of therings positioned on one side of the center of the inner volume isdifferent from the minimal radius of the rings positioned on the otherside of the center. The center of the inner volume can be defined in anysuitable way, e.g., the center of the section of the longitudinal axisthat lies within the inner volume. In addition, when the inner imagingvolume is only partially enclosed by the array the center will beconsidered as the center of the section of the longitudinal axis thatlies within the inner volume and inside the array. An array which obeysthe former embodiment may be described as comprised of twosub-assemblies with different minimal inner radiuses as described above.

Inner volume 130 is a simply-connected region at least partiallyenclosed by assembly 110, which is typically an ellipsoid or a sphere.As shown, the inner volume 130 is enclosed by the magnet array 110, withrings 112-113 encircling inner volume 130. In an embodiment, innervolume 130 is an oblate ellipsoid with semi-axes approximately equal to0.5 R1, 0.5 R1, and 0.3 R1. The parameters of such rings are not limitedto the inner and outer radius of a ring, its Z displacement, or Z-axisthickness. In addition, magnetic moment angles are all optimized using acalculation method such as a finite element, finite difference, oranalytical approach, combined with a gradient descent optimizationalgorithm to achieve the best uniformity, for a given field strength inthe imaging volume, with a minimal weight. This is allowed due to thefact that each assembly contains a multiplicity of rings, all of whichare optimized.

One aspect of the asymmetry of magnet array 100 is that different ringshave different transverse dimensions and magnetic moment directionswherein the rings are arranged in an array having reflectional asymmetrywith respect to the longitudinal axis (i.e., are asymmetrical withrespect to Z-axis inversion). In the context of the present disclosureand in the claims, the term “reflectional asymmetry with respect to thelongitudinal axis” means that no plane perpendicular to the longitudinalaxis is a plane of symmetry for the magnet array. In other words, themagnet array is not symmetric under flipping with respect to thelongitudinal axis at any point along the axis. Reflectional asymmetry isalso referred to as point asymmetry or mirror-image asymmetry. Forbrevity, any reference to “asymmetry” of the magnet array in thedescription below means the reflectional asymmetry defined above.

The asymmetry in the design is particularly advantageous when imaginginherently non-symmetrical specimens, such as the human head. Forexample, in one such case, it has been found that the rings belonging toassembly 110 may be primarily magnetized in a first given direction(e.g., the r-direction), whereas those belonging to assembly 120 mayprimarily magnetized in another direction (e.g., the z-direction).

Finally, the direction of magnetization of each individual ring may beoptimized to obtain both uniformity in the inner volume as well asfringe field reduction so as to create a magnetic circuit which closesthe field lines close to the magnet ring. In an embodiment, the discretemagnet segments are each pre-magnetized with a respective magnetizationdirection that minimizes a fringe field outside the magnet array.

FIGS. 2A and 2B-2D are a perspective view of an asymmetric magnet array200, and plots of magnetic field lines generated separately and jointlyby the assemblies, respectively, according to another embodiment of thepresent invention. Uniformity is not evident by uniform density of thelines (as lines were drawn denser in the imaging zone for betterdetails) rather by z-axis alignment of the lines.

As seen in FIG. 2A, an inner volume 230 is a simply-connected region atleast partially enclosed by a first magnet assembly 210, which istypically an ellipsoid or a sphere. A second magnet assembly 220 of theasymmetric array, “caps” inner volume 230. As mentioned above, differentrings may have different magnetization directions to optimize theuniformity and fringe field of the magnet array. For instance, one ringmay have a magnetization vector in a direction substantially different(e.g., by more than 45 degrees) from another ring. For instance, themagnetization vectors of the permanent magnet segments may pointprimarily in the r direction in one ring, and primarily in the Zdirection in another ring. Furthermore, two rings belonging to the sameassembly may have substantially different magnetization directions. Forinstance, one ring of the first assembly may have its magnetizationprimarily in the r direction, another ring of the first assembly mayhave its magnetization in primarily the −z direction while a third ringof the first assembly may have its magnetization at −45 degrees in ther-z plane. In an embodiment, the two or more ring have a magnetizationvector in a direction different by more than 45 degrees from oneanother.

In a particular case (not shown) it was found that the rings in assembly210 are dispersed in their inner radius between 15 cm and 30 cm, anddispersed in their Z position in a length of 25 cm, while the rings inassembly 220 are dispersed in their inner radius between 0.05 cm and 30cm, and dispersed in their Z position in a length of 12 cm, with thedisplacement between the two assemblies in the Z direction between 0 cmand 10 cm.

FIG. 2B shows the magnetic field lines of the field generated by firstmagnet assembly 210 (rings cross-sectionally illustrated by squares,each with a direction of magnetization of the ring in an r-z plane)inside and outside an inner volume 230. As seen, the field lines insideinner volume 230 are largely aligned along the z-axis, however theysharply bend at the top portion of volume 230, where the field becomesexceedingly non-uniform.

FIG. 2C shows the magnetic field lines of the field generated by secondmagnet assembly 220 inside and outside an inner volume 230. As also seenhere, the field lines inside inner volume 230 are largely aligned alongthe z-axis. However, they tilt opposite to the field lines of FIG. 2Bwith respect to the z-axis, and become exceedingly non-uniform at abottom portion of volume 230.

As seen on FIG. 2D, when combined into a full array 200, assemblies 210and 220 compensate for each other's field non-uniformity, to achieve auniform magnetic field along the z-axis to a better degree than aprespecified threshold.

FIGS. 2A-2D show an exemplary array containing ten rings. It will beappreciated that the array may contain more rings (e.g., several tens orhundreds of rings) which are all optimized as described above. The morerings contained in the array, the better magnet performance can beachieved (e.g., higher uniformity level, larger magnetic field or largerimaging volume). The improved performance comes with the drawback ofincreased complexity and production cost of the array due to the largenumber of elements. Thus, a practitioner skilled in the art shouldconsider the required number of rings according to the specificapplication.

FIG. 3 is a perspective view of a single segmented magnet ring 300,which may be any one of the rings in magnet arrays 100 and 200 of FIGS.1 and 2, according to an embodiment of the present invention. In FIG. 3,each magnet segment 310 has a magnetization vector 320 lying in the r-Zplane, with similar longitudinal (Z) and radial (r) components.Furthermore, each segmented ring possesses rotational symmetry with anazimuthal period equal to 360/N degrees where N is the number ofsegments in the ring. (For a solid ring, i.e., for N→∞, the rotationalsymmetry is continuous). In some embodiments, the disclosed rings haverotational symmetry of an order N≥8. It will be appreciated that thedisclosed array contains rings with rotational symmetry and hence theresult magnetic field is along the longitudinal axis. It is possiblehowever to incorporate in the asymmetric array rings which arenon-rotationally symmetric in a fashion that optimizes the fringe fieldand uniformity in the inner volume. In such a case the magnetic fieldmay be along an arbitrary axis. Although such an array may besubstantially worse than a rotationally symmetric array, the use ofasymmetry with rings as disclosed may substantially improve uniformityof the array compared to a symmetric one.

Discrete segments 310 are equally spaced and attached to one anotherusing, for example, an adhesive, which is preferably non-electricallyconducting, or are held together mechanically with gaps 330 betweenadjacent segments filled by (but not limited to) a preferably insulatingmaterial. It will be appreciated that the rotational symmetric segmentedrings may also include a combination of more than one type of segments.For thermal stability of all of ring 300, it is preferable that theadhesive or gaps consist of a material which is also thermallyconductive, such as silicon oxide, silicon nitride, or aluminum oxide.Individual magnet segments 310 may be made of the aforementionedstrongly ferromagnetic materials, whose Curie temperature is well abovethe operating temperature of an associated system that includes suchelements as an array 200, e.g., a mobile MRI system.

It will be appreciated that the descriptions in FIGS. 1-3 are intendedonly to serve as examples, and that many other embodiments are possiblewithin the scope of the present invention. For example, rotation of themagnet moment vector in the r-z-e plane can be achieved, in analternative embodiment, by rotating the individual magnet segments 310through a distinct angle of rotation, which may be different fordifferent rings. Further, magnet arrays 100 and 200 may be combined witheither a static or dynamic shimming system, to further improve fielduniformity inside inner volumes 130 and 230, respectively. When dynamicshimming or gradient pulse fields are used, the presence of electricallyinsulating adhesive or empty gaps between adjacent magnet segments 310helps to minimize the negative effects of eddy currents on fielduniformity. Furthermore, magnet arrays 100 and 200 may be combined withresistive coils placed concentric to the z-axis, in order to enhance themagnetic field strength inside inner volumes 130 and 230.

Magnet Array Including Mixed Phase Magnet Rings

FIG. 4 is a perspective drawing of a magnet array 400 ofphase-dissimilar mixed phase magnet rings (MPMRs), according to anembodiment of the present invention.

As seen, by way of example, array 400 comprises ten magnet rings 411-420which are coaxial with a central Z-axis passing through an inner volume430. The different rings are located at different positions along theZ-axis and, in general, have different transverse dimensions, radialthicknesses, and axial thicknesses. As seen, magnet array 400 hasreflectional asymmetry with respect to the longitudinal axis (i.e., isasymmetric with respect to Z-axis inversion). In the context of thepresent disclosure and in the claims, the term “reflectional asymmetrywith respect to the longitudinal axis” means that no plane perpendicularto the longitudinal axis is a plane of symmetry for the magnet array. Inother words, the magnet array is not symmetric under flipping withrespect to the longitudinal axis at any point along the axis.Reflectional asymmetry is also referred to as point asymmetry ormirror-image asymmetry. For brevity, any reference to “asymmetry” of themagnet array in the description below means the reflectional asymmetrydefined above.

In addition, inner volume 430 may be interior (as shown) or at leastpartially extending exterior in the z-direction (not shown) to magnetarray 400. Furthermore, the disclosed magnet array may or may not becombined with a yoke.

Ring 411 exemplifies an MPMR having cuboid-shaped permanent magnetelements (i.e. phase 1) separated by relatively small non-magnetic gaps(i.e. phase 2). Ring 413 exemplifies an MPMR having cuboid shapedpermanent magnet elements (i.e. phase 1) separated by relatively largenon-magnetic gaps (i.e. phase 2). Clearly, the fraction of the totalring volume occupied by non-magnetic gaps is small in the case of ring411 and relatively large in the case of ring 413. Thus, rings 411 and413 are MPMRs that are phase-dissimilar and array 400 may contain manyphase-dissimilar MPMRs.

Furthermore, ring 411 may also have a magnetization vector in adirection substantially different (e.g., by more than 45 degrees) fromring 413. For instance, the magnetization vectors of the permanentmagnet segments may point in the −Z direction in ring 411, and −45degrees in the r-Z plane in ring 413. In an embodiment, the two or moremixed-phase magnet rings contain only one magnetic phase with amagnetization vector in a direction different by more than 45 degreesfrom one another. Each MPMR ring possesses rotational symmetry with anazimuthal period equal to 360/N degrees where N is the number ofsegments in the ring. (For a continuous ring, i.e., for N→∞, therotational symmetry is continuous). In some embodiments, the disclosedMPMR rings have discrete rotational symmetry of an order N≥8.

FIG. 5 is a plot of magnetic field lines generated by magnet array 400of FIG. 4, according to an embodiment of the present invention.Uniformity is not evident by uniform density of the lines (as lines weredrawn denser in the imaging zone for better details) rather by z-axisalignment of the lines. MPMR array 400 can achieve, at a same arrayweight, a more uniform magnetic field along the z-axis, compared with,for example, that achieved with arrays 100 and 200. Moreover, theuniform field extends radially almost to the rings. Such lightweightMPMR arrays may be therefore particularly useful for mobile MRIapplications, such as an MRI ambulance.

FIGS. 4 and 5 show an exemplary array 400 containing ten MPMRs. It willbe appreciated that the array may contain more MPMRs (e.g., several tensor hundreds of MPMRs) which are all optimized as described above withmany of them phase dissimilar. The more rings contained in the array,the better magnet performance can be achieved (e.g., higher uniformitylevel, larger magnetic field or larger imaging volume). The improvedperformance comes with the drawback of increased complexity andproduction cost of the array due to the large number of elements. Thus,a practitioner skilled in the art should consider the required number ofMPMRs according to the specific application.

FIG. 6 is a perspective drawing of an exemplary MPMR 600 according to anembodiment of the invention. The ring consists of six repeating segments610, each of which has four elements: 620 a, 620 b, 620 c, and 620 d.

In an embodiment, elements 620 a are made of the aforementioned stronglymagnetic materials. Elements 620 a are typically pre-magnetized withspecific values for the components of magnetic moment. The shape ofelement 620 a may be cylindrical, as shown in FIG. 3, or some othershape such as a sphere, an ellipsoid, a cuboid, or a polygonal prism.

Element 620 c typically has a different phase from element 620 a. Forexample, it may have the same material composition and geometric shapeas element 620 a, but differ in one or more components of the magneticmoment, M. Alternatively, element 620 c may consist of anon-ferromagnetic material, such as a ferrimagnetic, paramagnetic, ornon-magnetic material, in which case the phase of element 620 c differsfrom that of element 620 a, by virtue of its different materialcomposition.

Element 620 b fills a gap of length L1 separating element 620 a fromelement 620 c; similarly, element 620 d fills a gap of length L2separating element 620 c from element 620 a of the adjacent segment, asshown in FIG. 6. Often, for thermal stability of MPMR 600, it ispreferable that elements 620 b and 620 d consist of non-magnetic,electrically non-conducting materials which are at least moderatelythermally conductive, such as silicon oxide, silicon nitride, oraluminum oxide.

In order to further illustrate the concept of phase-similar MPMR's,consider an MPMR 600 in which elements 620 a and 620 c have axialmagnetizations M0 and −M0, respectively. Next, consider a different MPMR600* (not shown) which is the same as MPMR 600 in all respects, exceptthat elements 620 a* and 620 c* have radial magnetizations 2M0 and −2M0,respectively. Since MPMR 600 can be transformed into MPMR 600* by acommon rotation of the magnetic moment by 90° in the r-Z plane followedby multiplication by a common scale factor of two, the two MPMR's areconsidered to be phase-similar. For each ring one may define theeffective strength of the ring by the magnitude of the volume averagedr-Z projection of magnetization vector divided by the largestmagnetization magnitude of all permanent magnet phases. The parameterhas a value between 0 and 1; and has the qualitative meaning of howeffective a ring produces a magnetic field nearby. When two rings arenot phase similar, they may have different relative effective strengthsand different contributions to the magnetic field.

Generally, adjacent elements in an MPMR are held together by mechanicalmeans or by adhesives. If the total volume occupied by adhesive layersis small, e.g. less than 1% of the total volume of the ring, then theadhesive layers need not be treated as an additional phase for thepurpose of magnetic field calculations. Small adjustments in thesegments positions and angles may be carried out to compensate for thesegments imperfections and residual inhomogeneity.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention. For example, magnet array 400 may becombined with either a static or dynamic shimming system to furtherimprove field uniformity inside inner volume 430. When dynamic shimmingor gradient pulse fields are used, the presence of electricallyinsulating material in the gaps between adjacent magnet elements helpsto minimize the deleterious effects of eddy currents on fielduniformity. Furthermore, magnet array 400 may be combined with resistivecoils placed concentric to the z-axis, in order to enhance the magneticfield strength inside inner volume 430.

In the example embodiments described herein, the mixed-phase rings arepart of an asymmetric magnet array. In alternative embodiments, however,mixed-phase rings may be used also in symmetric arrays or any other typeof magnet array, with or without a yoke to enhance their uniformity. Inaddition, the example magnet array described herein contains multiplerings coaxial with a common axis. It is however possible to combine thedescribed array with one or more additional ring arrays for which therings are coaxial with one or more different axes which are at an anglefrom the first longitudinal common axis. The combination of arraysjointly create a magnetic field in an arbitrary direction in space. Theadditional ring arrays may also contain mixed phase rings, those ringshowever are defined according to their own cylindrical coordinate systemwith a z′ axis defined as their own common coaxiality axis.

It is possible, for example, to have two arrays of rings with respectivecoaxiality axes that differ by 45 degrees from one another. Each arraymay contain two or more phase-dissimilar MPMRs and may be optimized toobtain a field substantially uniform in the inner volume along each ofthe array axis. The combination of the two arrays results in ahomogeneous magnetic field in a direction which is between the first andsecond longitudinal axes.

Although the embodiments described herein mainly address mobile MRIapplication, the methods and systems described herein can also be usedin other applications, such as aerospace applications, that requirestrong, uniform and lightweight magnets such as scanning electronmicroscopes (SEM).

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art. Documents incorporated by reference inthe present patent application are to be considered an integral part ofthe application except that to the extent any terms are defined in theseincorporated documents in a manner that conflicts with the definitionsmade explicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

The invention claimed is:
 1. A magnet array, comprising: multiple magnetrings, which are positioned along a longitudinal axis and coaxially withthe longitudinal axis, wherein at least two of the magnet rings comprisemixed-phase magnet rings that are phase-dissimilar, and wherein themagnet rings are arranged with reflectional asymmetry with respect tothe longitudinal axis, the multiple magnet rings configured to jointlygenerate a magnetic field along a direction parallel to the longitudinalaxis of at least a given level of uniformity inside a predefined innervolume; and a frame, which is configured to fixedly hold the multiplemagnet rings in place.
 2. The magnet array according to claim 1, whereintwo of the at least two mixed-phase magnet rings contain only a singlepermanent magnetic phase with a magnetization vector in a directiondifferent by more than 45 degrees from one another.
 3. The magnet arrayaccording to claim 1, wherein each magnet ring has a rotational symmetrywith respect to an in-plane rotation of the magnet ring around thelongitudinal axis.
 4. The magnet array according to claim 1, wherein atleast one of the magnet rings encircles the predefined inner volume, andwherein a minimal inner radius of the magnet rings positioned on oneside of a center of the inner volume along the longitudinal axis isdifferent from the minimal inner radius of the magnet rings positionedon the other side of the center of inner volume.
 5. The magnet arrayaccording to claim 1, wherein the inner volume is an ellipsoid ofrevolution around the longitudinal axis.
 6. The magnet array accordingto claim 1, wherein a given magnet ring is made of one of a single solidelement and an assembly of discrete magnet segments.
 7. The magnet arrayaccording to claim 6, wherein the magnet ring is pre-magnetized with arespective magnetization direction that maximizes uniformity of themagnetic field inside the inner volume.
 8. The magnet array according toclaim 6, wherein the magnet ring is pre-magnetized with a respectivemagnetization direction that minimizes a fringe field outside the magnetarray.
 9. The magnet array according to claim 6, wherein the discretemagnet segments are electrically insulated from each other.
 10. Themagnet array according to claim 6, wherein each of the discrete magnetsegments has a shape that is one of a shape of sphere, a cylinder, anellipsoid and a polygonal prism.
 11. The magnet array according to claim6, wherein the discrete magnet segments are separated of each other byat least one non-magnetic element comprising a solid, gas or liquid. 12.The magnet array according to claim 1, wherein each of the magnet ringshas a shape comprising one of an ellipse, a circle and a polygon. 13.The magnet array according to claim 1, wherein each of the mixed phaserings has a discrete rotational symmetry of at least an order eight. 14.The magnet array according to claim 1, and comprising one or moreadditional arrays of magnet rings, wherein the magnet rings in theadditional arrays are coaxial with respective longitudinal axes that areset at respective angles from the longitudinal axis.
 15. A method forproducing a magnet array, the method comprising: positioning multiplemagnet rings along a longitudinal axis and coaxially with thelongitudinal axis, wherein at least two of the magnet rings comprisemixed-phase magnet rings that are phase-dissimilar, and wherein themagnet rings are arranged with reflectional asymmetry with respect tothe longitudinal axis, the multiple magnet rings configured to jointlygenerate a magnetic field along a direction parallel to the longitudinalaxis of at least a given level of uniformity inside a predefined innervolume; and fixedly holding the multiple magnet rings in place using aframe.
 16. The method according to claim 15, wherein two of the at leasttwo mixed-phase magnet rings contain only a single permanent magneticphase with a magnetization vector in a direction different by more than45 degrees from one another.
 17. The method according to claim 15,wherein each magnet ring has a rotational symmetry with respect to anin-plane rotation of the magnet ring around the longitudinal axis. 18.The method according to claim 15, wherein at least one of the magnetrings encircles the predefined inner volume, and wherein a minimal innerradius of the magnet rings positioned on one side of a center of theinner volume along the longitudinal axis is different from the minimalinner radius of the magnet rings positioned on the other side of thecenter of inner volume.
 19. The method according to claim 15, whereinthe inner volume is an ellipsoid of revolution around the longitudinalaxis.
 20. The method according to claim 15, wherein a given magnet ringis made of one of a single solid element and an assembly of discretemagnet segments.
 21. The method according to claim 20, wherein themagnet ring is pre-magnetized with a respective magnetization directionthat maximizes uniformity of the magnetic field inside the inner volume.22. The method according to claim 20, wherein the magnet ring ispre-magnetized with a respective magnetization direction that minimizesa fringe field outside the magnet array.
 23. The method according toclaim 20, wherein the discrete magnet segments are electricallyinsulated from each other.
 24. The method according to claim 22, whereineach of the discrete magnet segments has a shape that is one of a shapeof sphere, a cylinder, an ellipsoid and a polygonal prism.
 25. Themethod according to claim 22, wherein the discrete magnet segments areseparated of each other by at least one non-magnetic element comprisinga solid, gas or liquid.
 26. The method according to claim 15, whereineach of the magnet rings has a shape comprising one of an ellipse, acircle and a polygon.
 27. The method according to claim 15, wherein eachof the mixed phase rings has a discrete rotational symmetry of at leastan order eight.
 28. The method according to claim 15, and comprisingpositioning one or more additional arrays of magnet rings, wherein themagnet rings in the additional arrays are coaxial with respectivelongitudinal axes that are set at respective angles from thelongitudinal axis.